1. Field of the Invention
The present invention relates to a method for synthesizing an organic substance using supercritical water, in particular, a method for synthesizing acrolein, which is a raw material for 1,3-propanediol, from glycerin in the presence of proton.
2. Background Art
Recently, demand for 1,3-propanediol has been increased because 1,3-propanediol is a raw material for high quality polyester fibers including polytrimethylene terephthalate. One of the methods for synthesizing 1,3-propanediol is a method for hydrating and hydrogenating acrolein shown in “Production, applications and economic efficiency of 1,3-PDO and PTT, CMC Co., Ltd., Planet Division, August, 2000.” This method produces 1,3-propanediol by hydration and hydrogenation reactions of acrolein obtained by air oxidation of propylene, which is a petroleum-based raw material, in the presence of a catalyst; this method is established as an industrial method. However, because of the recent increase in the price of crude oil, development of methods for synthesizing 1,3-propanediol from biological raw materials has been demanded.
There has not been reported any synthesis method of chemically synthesizing 1,3-propanediol from biological raw materials; however, there are techniques for synthesizing acrolein, which is a precursor of 1,3-propanediol, and examples of such techniques include a technique described in “WATANABE Masaru, IIDA Toru, AIZAWA Yuichi, AIDA Taku M, INOMATA Hiroshi, Acrolein synthesis from glycerol in hot-compressed water, Bioresource Technology 98, 1285-1290 (2007).” This method is a method in which, by using a small-scale apparatus such that the pipe diameter is of the order of 1 mm and the flow rate is 10 to 50 ml/min, an aqueous solution of glycerin as a biological raw material and high-temperature supercritical water are mixed with each other at 35 MPa, and thus the temperature of the resulting mixture is instantly increased to 400° C. to synthesize acrolein (the optimal reaction time is about 20 seconds). This method is characterized in that the proton originating from sulfuric acid added in a small amount to the aqueous solution of glycerin functions as a catalyst accelerating the dehydration reaction of glycerin. However, in “WATANABE Masaru, IIDA Toru, AIZAWA Yuichi, AIDA Taku M, INOMATA Hiroshi, Acrolein synthesis from glycerol in hot-compressed water, Bioresource Technology 98, 1285-1290 (2007),” the glycerin concentration in the raw material is as low as about 1%, and a large amount of energy is consumed for the temperature increase and pressure increase of water, and hence, for the purpose of commercial manufacturing, it is necessary to increase the glycerin concentration in the reaction solution to a high concentration of at least 15% or more.
However, when the glycerin concentration is increased to 15% or more, the reaction rate comes to be high and the optimal reaction time comes to be a few seconds, and hence complete mixing is required to be completed in at least 1/10th the reaction time. On the other hand, with the increase of the glycerin concentration, the viscosity difference between the supercritical water and the aqueous solution of glycerin is increased, and accordingly the miscibility therebetween is degraded. In particular, in a commercial plant of a size of a few ten thousands t/y, in the case where the reaction solutions are mixed at an economic flow speed, the pipe diameter comes to be about 1 to 10 cm, and concomitantly, the diffusion distance is also increased. In this connection, the mixing time is reciprocally proportional to the square of the pipe diameter, and hence the mixing time comes to be a few seconds or more. When the miscibility is degraded, the coordination number of the supercritical water in the vicinity of the glycerin molecules is degraded. FIG. 1 shows the dehydration reaction route of glycerin on the basis of the use of supercritical water. When the coordination number is decreased, the side reaction proceeds more predominantly than the main reaction to produce acrolein, and hence the reaction yield of acrolein is degraded. Additionally, with the decrease of the miscibility, glycerin is brought into contact with supercritical water to react with supercritical water at a temperature higher than the optimal reaction temperature, and hence the amounts of the generated reaction by-products such as tar and carbon particles are increased to further decrease the yield. The carbon particles aggregated with the aid of tar adhere to the valving elements and valve seats. Consequently, abrasion or the like of the valving elements and the valve seats occurs, and the operation ranges of the valving elements are limited to lead to a possibility that precise pressure control is made difficult. Therefore, from the viewpoints of the increase of the glycerin concentration and the scale-up of the reaction, the improvement of the miscibility is required.
In JP Patent Publication (Kokai) No. 2006-167600, a method for improving the miscibility is reported. In this method, the introduction pipe of a first fluid and the introduction pipe of a second fluid are connected to the mixing pipe under the condition that the central axis of the introduction pipe of the first fluid and the central axis of the introduction pipe of the second fluid are deviated from each other, and thus swirl flow is generated in the mixing pipe to thereby improve the miscibility. However, the number of the introduction pipes is small, and hence a high miscibility is obtained with the thin pipe of the order of millimeters in the diameter of the mixing pipe, but in a case of a commercial plant of the order of a few ten thousands t/y having a mixing pipe diameter of the order of 10 cm, no sufficient miscibility is obtained.
On the other hand, in the high-temperature, high-pressure micromixer described in JP Patent Publication (Kokai) No. 2008-12453, a first reaction solution is introduced into the central axis of a mixing pipe and two introduction pipes of a second reaction solution are disposed at the positions offset from the central axis, and hence there is a problem that a multiple layer flow is hardly formed and the mixing time is made long.